1. Field of the Invention
The present invention relates to a dielectric filter. More specifically, the present invention relates to a dielectric filter in which a capacitor or an inductance is connected in parallel to a dielectric resonator by means of a switch, so as to provide multiple frequencies by the switching of the switch.
2. Description of the Background Art
FIG. 1 shows a conventional dielectric filter. Referring to FIG. 1, coupling capacitors C1, C2 and C3 are connected in series between input and output terminals, a resonator R1 is connected between a node between coupling capacitors C1 and C2 and ground, and a resonator R2 is connected between a node between coupling capacitors C2 and C3 and ground.
FIG. 2 shows the frequency-amplitude characteristic of the dielectric resonator shown in FIG. 1. The bandpass filter characteristic of the dielectric filter shown in FIG. 1 has one central frequency f.sub.0, as shown in FIG. 2. Therefore, a different dielectric filter cannot freely select the central frequency.
FIG. 3 is a schematic diagram showing a conventional, duplex-type dielectric filter. The duplex-type dielectric filter shown in FIG. 3 includes a transmitting-side dielectric filter 11, a receiving-side dielectric filter 12, a matching circuit 13 connected between the transmitting-side filter 11 and an antenna terminal ANT, and a matching circuit 14 connected between the receiving-side dielectric filter 12 and antenna terminal ANT. Transmitting-side dielectric filter 11 is structured in a manner similar to that of the dielectric filter shown in FIG. 1. Receiving-side dielectric filter 12 is also structured in the same manner, and includes coupling capacitors C11, C12 and C13, as well as resonators R3 and R4.
In the duplex-type dielectric filter shown in FIG. 3, the characteristic of the transmitting-side dielectric filter 11 and the receiving-side dielectric filter 12 has one central frequency f.sub.0 as shown in FIG. 2. It is necessary to provide two bandpass filter circuits, that is, transmitting-side and receiving-side dielectric filters 11 and 12, and also to provide matching circuits 13 and 14 for matching these with the impedance of the antenna. Therefore, the required number of parts is considerably large.